1. Field of the Invention
The present invention relates to a unit configured to detect the amount of misregistration between multiple color images in the case of obtaining an image visualized by superposing multiple colors in a tandem-type image forming apparatus.
2. Description of the Related Art
According to a so-called “tandem” image forming apparatus, a different image forming part is used for each of four colors, and a (composite) color image is formed by superposing toner images directly on paper or on an intermediate transfer belt.
Such a tandem image forming apparatus cannot produce stable color images if the color toner images are superposed at slightly different positions. Therefore, such misregistration correction is commonly performed as detecting misregistration correction patterns for the respective colors formed on the conveyor body (conveyor belt) of the image forming apparatus and superposing the four colors on the same position. In general, the results of detection of color patterns (cyan, magenta, and yellow) and the result of detection of a reference color pattern (black) are compared, and the amounts of misregistration of the color patterns relative to the reference color pattern are calculated. (See, for example, Japanese Laid-Open Patent Application No. 2003-266798 and Japanese Patent No. 3186587.)
On the other hand, since the conveyor belt, on which the misregistration correction patterns are formed, is not uniform in thickness, the surface velocity of the conveyor belt varies to cause detection error.
Therefore, in order to cancel the effect of the above-described detection error due to the non-uniform thickness of the conveyor belt, misregistration correction patterns are formed on the conveyor belt over its overall length or more (for one turn or more), and then the amount of misregistration is detected.
However, forming misregistration correction patterns on the conveyor belt over its overall length or more and detecting the amount of registration as described above cause the problem of a longer time for forming and detecting the misregistration correction patterns.